On Mon, Nov 07, 2016 at 04:43:13PM +0000, Esparza Borquez, David wrote: > We are measuring virtio blk latency in linux containers following the method presented by you in > http://www.linux-kvm.org/page/Virtio/Block/Latency and we have some questions: That page is old but the general approach still makes sense. Some of the details (e.g. trace event names) have changed. I also recommend comparing against blktrace and benchmark I/O statistics to sanity-check your tracing results. That way you can be sure that the numbers you're getting make sense. > 1. Does a single paio_submit call is followed by a pair of virtio_queue_notify/vitio_notify ? > or a single paio_submit call could be followed by several virtio_queue_notify/vitio_notify traces. It should be: 1 x virtio_queue_notify() N x paio_submit() (N >= 1, there could be multiple requests to submit) N x virtio_notify() Note that the N virtio_notify() calls could happen at any time, even after the *next* virtio_queue_notify(). This is because virtio-blk requests can be done in parallel and their lifetimes can overlap. Note that paio_submit() is used with the defailt -drive aio=threads option. If you use -drive aio=native then laio_submit() can be used instead. > 2. Sometimes Host kvm.ko (kvm_pio:pio_write) traces appear to have less latency than QEMU paio/virtio traces and in theory they should be grater... do you think this is due to the clock we are using in the host/guest? we've tried several combinations of clocks but so far no luck or at least none has been consistent. Which tracing backend did you enable in QEMU ./configure? I'd expect kvm:kvm_pio and QEMU trace events to use coherent clock sources. > 3. Because we have not observed results for QEMU paio latency when tracking posix_aio_process_queue( ) , we opted for measuring 'paio' latency but with the event 'paio_submit'. Do you think that could be ok to measure qemu paio latency? posix_aio_process_queue does not exist in modern QEMU. Latency is the time between 2 events. You have only mentioned paio_submit so I don't know what other event you are using to calculate the latency.
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